Gymnarchus niloticus – commonly known as the aba, aba aba, frankfish, freshwater rat-tail, poisson-cheval, or African knifefish – is an electric fish, and the only species in the genus Gymnarchus and the family Gymnarchidae within the order Osteoglossiformes. It is found exclusively in swamps and near vegetated edges in the Nile, Turkana, Chad, Niger, Volta, Senegal, and Gambia basins.
Description and biology
G. niloticus has a long and slender body, with no caudal, pelvic, or anal fins. The dorsal fin is elongated, running along the back of the fish towards the blunt, finless tail, and is the main source of propulsion. It grows up to 1.6 m (5.2 ft) in length and 19 kg (42 lb) in weight.
G. niloticus is nocturnal and has a poor vision. Instead, it navigates and hunts smaller fish using a weak electric field similar to that of the related elephantfish. Also like the elephantfish, it possesses an unusually large brain, which is believed to help it interpret the electrical signals. It can make its tail negatively charged with respect to its head. This produces a symmetrical electric field around its body. Nearby objects distort this field, and it can sense the distortion on its skin.
G. niloticus females lay their eggs in floating nests up to a metre across. The adults continue to guard the young after hatching.